Fix up handling for awaitable IResults in ApiExplorer

src/Mvc/Mvc.ApiExplorer/src/ApiResponseTypeProvider.cs

@@ -228,6 +228,13 @@ internal static Dictionary<int, ApiResponseType> ReadResponseMetadata(
 
         foreach (var metadata in responseMetadata)
         {
+            // `IResult` metadata inserted for awaitable types should
+            // not be considered for response metadata.
+            if (typeof(IResult).IsAssignableFrom(metadata.Type))
+            {
+                continue;
+            }
+
             var statusCode = metadata.StatusCode;
 
             var apiResponseType = new ApiResponseType

src/Mvc/Mvc.ApiExplorer/src/EndpointMetadataApiDescriptionProvider.cs

@@ -324,12 +324,6 @@ private static void AddSupportedResponseTypes(
     {
         var responseType = returnType;
 
-        // Can't determine anything about IResults yet that's not from extra metadata. IResult<T> could help here.
-        if (typeof(IResult).IsAssignableFrom(responseType))
-        {
-            responseType = typeof(void);
-        }
-
         // We support attributes (which implement the IApiResponseMetadataProvider) interface
         // and types added via the extension methods (which implement IProducesResponseTypeMetadata).
         var responseProviderMetadata = endpointMetadata.GetOrderedMetadata<IApiResponseMetadataProvider>();
@@ -338,6 +332,14 @@ private static void AddSupportedResponseTypes(
         var defaultErrorType = errorMetadata?.Type ?? typeof(void);
         var contentTypes = new MediaTypeCollection();
 
+        // If the return type is an IResult or an awaitable IResult, then we should treat it as a void return type
+        // since we can't infer anything without additional metadata.
+        if (typeof(IResult).IsAssignableFrom(responseType) ||
+            producesResponseMetadata.Any(metadata => typeof(IResult).IsAssignableFrom(metadata.Type)))
+        {
+            responseType = typeof(void);
+        }
+
         var responseProviderMetadataTypes = ApiResponseTypeProvider.ReadResponseMetadata(
             responseProviderMetadata, responseType, defaultErrorType, contentTypes, out var errorSetByDefault);
         var producesResponseMetadataTypes = ApiResponseTypeProvider.ReadResponseMetadata(producesResponseMetadata, responseType);

src/Mvc/Mvc.ApiExplorer/test/EndpointMetadataApiDescriptionProviderTest.cs

@@ -8,6 +8,7 @@
 using System.Security.Claims;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.HttpResults;
 using Microsoft.AspNetCore.Http.Metadata;
 using Microsoft.AspNetCore.Mvc.Abstractions;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
@@ -273,6 +274,144 @@ public void AddsMultipleResponseFormatsFromMetadataWithIResult()
         Assert.Empty(badRequestResponseType.ApiResponseFormats);
     }
 
+    [Fact]
+    public void AddsMultipleResponseFormatsForTypedResults()
+    {
+        var apiDescription = GetApiDescription(Results<Created<InferredJsonClass>, BadRequest> () =>
+            TypedResults.Created("https://example.com", new InferredJsonClass()));
+
+        Assert.Equal(2, apiDescription.SupportedResponseTypes.Count);
+
+        var createdResponseType = apiDescription.SupportedResponseTypes[0];
+
+        Assert.Equal(201, createdResponseType.StatusCode);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.Type);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.ModelMetadata?.ModelType);
+
+        var createdResponseFormat = Assert.Single(createdResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdResponseFormat.MediaType);
+
+        var badRequestResponseType = apiDescription.SupportedResponseTypes[1];
+
+        Assert.Equal(400, badRequestResponseType.StatusCode);
+        Assert.Equal(typeof(void), badRequestResponseType.Type);
+        Assert.Equal(typeof(void), badRequestResponseType.ModelMetadata?.ModelType);
+
+        Assert.Empty(badRequestResponseType.ApiResponseFormats);
+    }
+
+    [Fact]
+    public void AddsResponseFormatsForTypedResultWithoutReturnType()
+    {
+        var apiDescription = GetApiDescription(() => TypedResults.Created("https://example.com", new InferredJsonClass()));
+
+        Assert.Equal(1, apiDescription.SupportedResponseTypes.Count);
+
+        var createdResponseType = apiDescription.SupportedResponseTypes[0];
+
+        Assert.Equal(201, createdResponseType.StatusCode);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.Type);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.ModelMetadata?.ModelType);
+
+        var createdResponseFormat = Assert.Single(createdResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdResponseFormat.MediaType);
+    }
+
+    [Fact]
+    public void HandlesTypedResultsWithoutIEndpointMetadataProviderImplementation()
+    {
+        // TypedResults for ProblemDetails doesn't implement IEndpointMetadataProvider
+        var apiDescription = GetApiDescription(() => TypedResults.Problem());
+
+        Assert.Equal(1, apiDescription.SupportedResponseTypes.Count);
+
+        var responseType = apiDescription.SupportedResponseTypes[0];
+
+        Assert.Equal(200, responseType.StatusCode);
+        Assert.Equal(typeof(void), responseType.Type);
+        Assert.Equal(typeof(void), responseType.ModelMetadata?.ModelType);
+    }
+
+    [Fact]
+    public void AddsResponseFormatsForAwaitableTypedResultWithoutReturnType()
+    {
+        var apiDescription = GetApiDescription(() =>
+            Task.FromResult(TypedResults.Created("https://example.com", new InferredJsonClass())));
+
+        Assert.Equal(1, apiDescription.SupportedResponseTypes.Count);
+
+        var createdResponseType = apiDescription.SupportedResponseTypes[0];
+
+        Assert.Equal(201, createdResponseType.StatusCode);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.Type);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.ModelMetadata?.ModelType);
+
+        var createdResponseFormat = Assert.Single(createdResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdResponseFormat.MediaType);
+    }
+
+    // Coverage for https://github.com/dotnet/aspnetcore/issues/56975
+    [Fact]
+    public void AddsMultipleResponseFormatsFromMetadataWithAwaitableIResult()
+    {
+        var apiDescription = GetApiDescription(
+            [ProducesResponseType(typeof(InferredJsonClass), StatusCodes.Status201Created)]
+        [ProducesResponseType(StatusCodes.Status400BadRequest)]
+        () => Task.FromResult(Results.Ok(new InferredJsonClass())));
+
+        Assert.Equal(2, apiDescription.SupportedResponseTypes.Count);
+
+        var createdResponseType = apiDescription.SupportedResponseTypes[0];
+
+        Assert.Equal(201, createdResponseType.StatusCode);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.Type);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.ModelMetadata?.ModelType);
+
+        var createdResponseFormat = Assert.Single(createdResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdResponseFormat.MediaType);
+
+        var badRequestResponseType = apiDescription.SupportedResponseTypes[1];
+
+        Assert.Equal(400, badRequestResponseType.StatusCode);
+        Assert.Equal(typeof(void), badRequestResponseType.Type);
+        Assert.Equal(typeof(void), badRequestResponseType.ModelMetadata?.ModelType);
+
+        Assert.Empty(badRequestResponseType.ApiResponseFormats);
+    }
+
+    [Fact]
+    public void AddsMultipleResponseFormatsFromMetadataWithAwaitableResultType()
+    {
+        var apiDescription = GetApiDescription(
+            [ProducesResponseType(typeof(InferredJsonClass), StatusCodes.Status201Created)]
+        [ProducesResponseType(StatusCodes.Status400BadRequest)]
+        async Task<Results<Created<InferredJsonClass>, ProblemHttpResult>> () => {
+            await Task.CompletedTask;
+            return Random.Shared.Next() % 2 == 0
+                ? TypedResults.Created<InferredJsonClass>("/", new InferredJsonClass())
+                : TypedResults.Problem();
+        });
+
+        Assert.Equal(2, apiDescription.SupportedResponseTypes.Count);
+
+        var createdResponseType = apiDescription.SupportedResponseTypes[0];
+
+        Assert.Equal(201, createdResponseType.StatusCode);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.Type);
+        Assert.Equal(typeof(InferredJsonClass), createdResponseType.ModelMetadata?.ModelType);
+
+        var createdResponseFormat = Assert.Single(createdResponseType.ApiResponseFormats);
+        Assert.Equal("application/json", createdResponseFormat.MediaType);
+
+        var badRequestResponseType = apiDescription.SupportedResponseTypes[1];
+
+        Assert.Equal(400, badRequestResponseType.StatusCode);
+        Assert.Equal(typeof(void), badRequestResponseType.Type);
+        Assert.Equal(typeof(void), badRequestResponseType.ModelMetadata?.ModelType);
+
+        Assert.Empty(badRequestResponseType.ApiResponseFormats);
+    }
+
     [Fact]
     public void AddsFromRouteParameterAsPath()
     {